In a cellular communications network, user equipment (UE) (such as mobile telephones, mobile devices, mobile terminals, etc.) can communicate with other user equipment and/or remote servers via base stations. LTE systems include an Evolved Universal Terrestrial Radio Access Network (E-UTRAN) and an Evolved Packet Core (EPC) network (or simply ‘core network’). The E-UTRAN includes a number of base stations (‘eNBs’) for providing both user-plane and control-plane terminations towards the UE.
Depending on various criteria (such as the amount of data to be transmitted, radio technologies supported by the mobile telephones, expected quality of service, subscription settings, etc.), each base station is responsible for controlling the transmission timings, frequencies, transmission powers, modulations, etc. employed by the mobile telephones attached to the base station. In order to minimise disruption to the service and to maximise utilisation of the available bandwidth, the base stations continuously adjust their own transmission power and also that of the mobile telephones. Base stations also assign frequency bands and/or time slots to mobile telephones, and also select and enforce the appropriate transmission technology to be used between the base stations and the attached mobile telephones. By doing so, base stations also reduce or eliminate any harmful interference caused by mobile telephones to each other or to the base stations.
A so-called Downlink Coordinated Multi-Point (CoMP) transmission/reception feature was introduced in Rel-11 of the 3GPP standards documentation to improve, for example, the coverage of high data rates for user equipment, temporary network deployment, cell edge throughput and/or to increase system throughput. The CoMP feature established techniques for compatible mobile telephones (and other user equipment) to communicate with multiple transmission points (TPs), substantially simultaneously. The TPs typically include: base stations (eNBs), remote radio heads (RRHs), relay nodes (RNs), and/or the like (or combination thereof). The transmission points involved in CoMP transmission may be provided in a number of ways including, for example: as separate transmission points of a single base station, each transmission point being associated with a different cell (referred to as ‘intra-eNB CoMP’); as transmission points operated by different base stations (referred to as ‘inter-eNB CoMP’), or using a combination of these two methods. These techniques are described in, for example, TR 36.819 V11.2.0, the contents of which are hereby incorporated by reference. In summary, CoMP transmission/reception may be used i) to optimise received signal quality at the mobile telephone by transmitting the same signal from multiple TPs and/or ii) to increase data throughput by sending different signals (e.g. different parts of the user data) from different TPs concurrently (but of course without causing interference, e.g. by using different frequencies/timing/codes/etc).
However, when multiple transmission points are used by the mobile telephone, it is often difficult to find an optimal combination of cells and to determine an optimum scheduling in each cell (i.e. that maximises data rate and/or avoids unnecessary interference). Furthermore, base stations are often connected via a so-called non-ideal backhaul, characterised by a relatively high latency and/or limited bandwidth, which often causes data transmitted between such base stations to suffer delays (sometimes 50 ms or more) that may inhibit efficient CoMP operation. CoMP scheduling algorithms typically require the exchange of large amounts of data between base stations (cells), which is not practical for (at least) inter-eNB CoMP transmission because of the above mentioned backhaul restrictions.
It has been proposed that CoMP scheduling can be performed in two distinct stages (at a radio frame level and at a sub-frame level, respectively) such that it requires only a limited coordination among TPs in the coordination area. It is therefore possible to realise CoMP scheduling using TPs connected via a non-ideal backhaul.
In the first stage, performed for each radio frame (which has a duration of 10 ms in LTE), a central controller selects which TP is to be made active for which UE (i.e. which TP is turned ‘ON’ during that radio frame). This is called associating between UEs and TPs. The first stage is performed periodically at a relatively coarse granularity based on averaged (not instantaneous) slowly varying metrics that are relevant for a period longer than the backhaul latency (typically ranging from 50 ms—equivalent to five LTE radio frames—to 100 ms or even greater). Thus, the metrics being used do not change significantly while the signals, from which the metrics are extracted, are being communicated via the backhaul. Examples of such slowly varying metrics include estimates of average rates that the users can receive from those TPs under different configurations and/or the like. Such metrics may be based, for example on relatively long-term measurements of reference received power (RSRP) made by each UE.
On the other hand, the second stage is performed for each sub-frame (having a duration of 1 ms in LTE) and/or each slot (0.5 ms) independently by each active TP based on fast changing information, such as instantaneous rate or signal-to-interference-plus-noise ratio (SINR) estimates, that is received directly by that TP from the mobile telephones associated to it. Therefore, the second stage does not require coordination between the TPs involved, at least until the end of the current radio frame (end of the first stage).
However, such an approach is not ideal because it may result in a sub-optimal CoMP configuration (UE-TP associations) being selected for the duration of several radio frames, which in turn may result in a poor overall data throughput and/or unwanted interference for the duration of the entire set of radio frames. Such a sub-optimal CoMP configuration cannot be compensated for (or alleviated by) the active TPs in the second stage (even if it is mainly caused by a single TP) because in the second stage the TPs are operating independently from each other and hence they can only change the parameters of their own transmission in each sub-frame (or slot).